Although mixing light seems simple on the surface, it can be really
confusing. Some people dispute whether the things you see on the
screen really represent shadows or not. Whatever you call them, this is what happens.

Suppose you have a red light and a green light. You see this
(without the letters!):

It looks as if the green light casts a red shadow and the red one
casts a green shadow. What's going on?

Try to think about it one region at a time. For example,
both the red and the green lights are shining on region D (the yellow
part). It may seem odd, but in light, red and green mix together to
make yellow.

Neither light can get to region C, so it shows up black.

This leaves regions A and B. In region A, the red light is shining but the
green is blocked by the card. Here's another way to think of this:
From region A, you can see the red light bulb but not the green one.
Just the opposite is true for region B.

About mixing, pigment is different from light.

You are probably used to mixing colors as pigmentslike paints or
food coloring. In such situations, colors subtract. The more
colors you put in, the less color you get; that is, you get closer to
black, the absence of color. In pigments, for example, blue and yellow
make green.

Lights are different. When you put more light in, the colors
add. The more colors you put in, the closer you get to white,
the mixture of all colors. With light, though it seems weird, red and
green make yellow. The other two combinations are more intuitive: Red
and blue make MAGENTA; blue and
green make CYAN.

The reason for the colored shadows may be too hard for very young
children, but they can still reason about mixing colors and see the
patterns that emerge.

Older children can ask even more sophisticated questions and
design experiments or demonstrations to answer them, for example:

What happens if there are three different-colored lamps
instead of two?

How can it be that light and pigment have different
primary colors?

What geometrical shapes do the shadows make?

The National Science Education Standards (1996) state that
"as a result of the activities in grades K4, all students
should develop an understanding of light...." (p. 123). The
Standards state specifically that "By experimenting with light...,
students begin to understand that phenomena can be observed,
measured, and controlled in various ways" (p. 126).